This application claims the benefit of a Japanese Patent Application No. 2001-235080 filed Aug. 2, 2001, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.
1. Field of the Invention
The present invention generally relates to smoothing methods, smoothing circuits and image output apparatuses, and more particularly to a smoothing method and a smoothing circuit which are suited for printers and display units which treat multi-value (or multi-level) image data, and to an image output apparatus which employs such a smoothing method and smoothing circuit.
The multi-value image data generated by a computer or the like is output to an image output apparatus such as the printer and the display unit. Particularly in the case of an image made up of characters and line drawings, a smoothing process is carried out as an image quality improving process to make jaggy portions which are caused by pixels (dots) to become less conspicuous.
2. Description of the Related Art
FIG. 1 is a diagram showing the construction of an example of a conventional laser beam printer. The laser beam printer generally includes a main body 1, an image memory 12, an image developing section 13, a control circuit 14, and an optical modulating signal generating circuit 15.
The main body 1 includes an optical section 2 and an image forming section 11. The optical section 2 includes a laser diode 3 which emits a laser beam, a polygonal mirror 4 which deflects the laser beam so as to make repeated scans, a mirror motor 5 which rotates the polygonal mirror 4, and a beam detector 6 which detects a start of the scan of the laser beam. The image forming section 11 includes a photoconductive drum 7, a developing unit 8, a transfer roller 10, and a mirror 9.
The surface of the photoconductive drum 7 is charged by a charger (not shown), and the laser beam thereafter irradiates the charged surface. The laser diode 3 is modulated in synchronism with the scan of the laser beam and the rotation of the photoconductive drum 7, so that an optical image corresponding to the image to be printed is formed on the surface of the photoconductive drum 7. The electrostatic charge on the surface of the photoconductive drum 7 decreases depending on the amount of irradiation of the laser beam, to form an electrostatic latent image. When toner is supplied from the developing unit 8 to the charged surface of the photoconductive drum 7, the toner adheres on the surface of the photoconductive drum 7 depending on the electrostatic latent image, to thereby visualize the electrostatic latent image into a toner image. A recording medium such as paper is transported to contact the surface of the photoconductive drum 7 having the toner image, and the toner image is transferred onto the recording medium by the transfer roller 10. The toner image on the recording medium is fixed by a fixing unit (not shown), and the printing ends. The surface of the photoconductive drum 7 after the toner image is transferred onto the recording medium is cleaned, before the surface of the photoconductive drum 7 is again charged to repeat the above described process.
The print data received from a computer system (not shown) or the like is developed into print image data by the image developing section 13 and stored in the image memory 12. Generally, the image memory 12 is referred to as a bit-map memory, and in the case of a binary print data, each bit of the bit-map memory, that is, one bit, corresponds to a printing pixel. In the case of a multi-value print data, several bits of the bit-map memory correspond to the printing pixel. For example, in the case of a 4-bit print data, the bit-map memory used has a 4-bit structure. The 4-bit print data can represent 16 gradation levels in units of pixels. The optical modulating signal generating circuit 15 reads the image data stored in the image memory 12 in synchronism with the process in the main body 1, and generates an optical modulating signal which is supplied to the laser diode 3.
In a single-color binary printing apparatus such as a monochromatic laser beam printer, the jaggy generated at the time of printing the characters and line drawings from the print bit map data developed in the image memory 12 is judged automatically, and the input image data is converted into an image data having a higher resolution that the input image data, before carrying out the smoothing process to make the jaggy less conspicuous.
FIGS. 2A through 2C are diagrams for explaining the smoothing process with respect to the binary image data. For example, if the print image data stored in the image memory 12 includes a vertical line which is deviated by one pixel (dot) at an intermediate portion thereof as shown in FIG. 2A, this deviated portion corresponds to the jaggy described above. The smoothing process judges the jaggy by carrying out a pattern matching including the neighboring pixel data. In addition, an output timing of the optical modulating signal is adjusted as shown in FIG. 2B at the jaggy portion. As a result, the printing is carried out in a state shifted by one-half dot at the jaggy portion, as shown in FIG. 2C, so that the vertical line changes smoothly, so that the jaggy is reduced. In this particular case, the image data is shifted by one-half dot in the same direction, but the shift may be made either to the right or left. In other words, the smoothing process enables the printing at a resolution which is an integral multiple of that of the original image data in a main scanning direction of the laser beam, and carries out a process of adding or removing dots at the jaggy portion.
FIG. 3 is a system block diagram showing an example of the construction of a smoothing circuit which carries out a smoothing process with respect to binary print data. As shown in FIG. 3, the optical modulating signal generating circuit 15 includes an image memory read section 16 and a smoothing circuit 17. Further, the smoothing circuit 17 includes a line buffer 18, an evaluation window extracting section 21, and a correction signal generating section 22.
The image memory read section 16 reads from the image memory 12 the bit-map data of several lines before the print data which is being exposed by the laser diode 3, and transfers the bit-map data to the line buffer 18. The line buffer 18 is made up of a shift register, and holds the data of several lines before and after the print data which is being exposed.
The evaluation window extracting section 21 extracts the data of a rectangular region (hereinafter referred to as an “evaluation window”) 19 having one target pixel (dot) 20 from the data held in the line buffer 18, and outputs an extracted pattern arrangement signal which indicates the pixel arrangement of the evaluation window 19. The extracted pattern arrangement signal is input to the correction signal generating circuit 22. The correction signal generating circuit 22 generates a correction signal (correction value) with respect to the target pixel 20, based on the dot arrangement within the evaluation window 19 indicated by the extracted pattern arrangement signal. The correction signal generating circuit 22 includes a look-up table which stores various kinds of patterns in a vicinity with respect to the target pixel 20, and collates the input extracted pattern arrangement signal and the look-up table, and outputs a correction signal which is stored in correspondence with the collated result. For example, if no correction is required, the modulating signal is generated based on the pixel data as it is and output as the correction signal. On the other hand, if correction is required, the correction signal is generated based on the prestored correction data.
FIGS. 4A through 4D are diagrams for explaining the smoothing process with respect to the pixel data having the dot arrangement pattern shown in FIG. 2A. FIG. 4A shows the pattern of the extracted evaluation window for a case where the pixel at the line N and the pixel position M−1 is regarded as the “target pixel”, and the “target pixel” and the neighboring pixels are extracted with a window size of 5×5 pixels. When it is judged that the print data and such a pattern stored in the look-up table match, it is judged that the correction of the pixel at the line N and the pixel position M−1 is required, and the correction data shown in FIG. 4B is output. The optical modulating signal is generated based on this correction data. As will be described later, the correction data is also generated with respect to the pixel at the line N and the pixel position M−2, and the optical modulating signal is generated based on the correction data.
FIG. 4C shows the pattern of the extracted evaluation window for a case where the pixel at the line N and the pixel position M+1 is regarded as the “target pixel”, and the “target pixel” and the neighboring pixels are extracted with a window size of 5×5 pixels. When it is judged that the print data and such a pattern stored in the look-up table match, it is judged that the correction of the pixel at the line N and the pixel position M+1 is required, and the correction data shown in FIG. 4D is output. The optical modulating signal is generated based on this correction data.
In the smoothing circuit shown in FIG. 3, the “target pixel” is successively moved in synchronism with the pixel print timing in the main body 1, and the optical modulating signal of the “target pixel” position is output when printing the pixel located at the position actually corresponding to the “target position”. A control clock for controlling this operation timing is generated by a control clock signal generating section 23 within the control circuit 14.
The description given heretofore applies to the case where the pixels are corrected by doubling the resolution in the main scanning direction. However, the process can be carried out similarly when correcting the pixels to make the resolution in the main scanning direction to become three or more times.
Multi-value printing apparatuses have also been proposed, which can represent half-tone pixels in units of printing pixels, in addition to the black-and-white binary pixels. For example, in the case of the laser beam printer shown in FIG. 1, it is possible to vary the size of the printing pixel formed by the image forming section 11 and equivalently vary the tone of the printing pixel, by controlling the light emission amount or the light emission time of the laser diode 3 of the optical section 2.
FIG. 5 is a system block diagram showing the construction of an example of a laser beam printer which carries out the multi-value printing in units of printing pixels, by varying the light emission amount of the laser diode. In this case, the main body 1 has the same construction as that shown in FIG. 1. The multi-value printing bit-map data having tone gradation is developed in the image memory 31, and an image memory read section 33 within the optical modulating signal generating circuit 32 reads the pixel data from the image memory 31 in synchronism with the pixel print timing in the image forming section 11. The pixel data read from the image memory 31 is converted by a digital-to-analog (D/A) converter 34 into the optical modulating signal having an analog intensity corresponding to the multi-value data depending on the tone value of the pixel data.
The smoothing process is also carried out in the laser beam printer which carries out the multi-value printing. FIGS. 6A through 6C are diagrams for explaining an example of the smoothing process with respect to a 4-value image data. FIG. 6A shows an example of the pattern of the 4-value bit-map data in the image memory 31, FIG. 6B shows the optical modulating signal for the case where the multi-value printing is carried out in units of printing pixels by varying the laser light emission amount, and FIG. 6C shows the printed result.
A smoothing circuit having a construction similar to that shown in FIG. 3 is used when carrying out the smoothing process with respect to the multi-value data shown in FIGS. 6A through 6C, and the only difference is that the pixel data is the multi-value data. For this reason, the line buffer 18 must be capable of storing the multi-value data, the evaluation window extracting section 21 must be capable of extracting and transferring the extracted pattern arrangement signal of the multi-value data to the correction signal generating section 22, and the correction signal generating section 22 must include a multi-value look-up table. However, if the number of tones including white that can be represented by the printing pixel unit is denoted by C, the number of combinations of the patterns which are extracted as the evaluation window becomes (C/2)25 times the case of binary data. In addition, since the correction signal (correction value) also represents C tones, the size of the look-up table further becomes D times, where D is an integer satisfying a condition 2D−1<C≦2D.
Therefore, when the conventional smoothing circuit is applied as it is to the smoothing process of the multi-value image in the multi-value printing apparatus, there are problems in that the size of the required look-up table becomes extremely large, and it is complicated to create the table data because of the large number of combinations of the pixel arrangement and the tone.
In order to eliminate these problems, a multi-value smoothing method was previously proposed in a Japanese Laid-Open Patent Application No. 11-319957. In this previously proposed multi-value smoothing method, the input multi-value image data is decomposed into a plurality of tone planes depending on the tone that can be represented in units of pixels, and the arrangement of the pixels is corrected by combining correction signals which are output as results of the smoothing process for each of the tone planes, as shown in FIGS. 7 and 8. FIG. 7 is a system block diagram showing the construction of an example of the smoothing circuit which carries out the smoothing process with respect to the multi-value print data. Further, FIG. 8 is a diagram for explaining the smoothing process with respect to the multi-value print data.
The laser beam printer shown in FIG. 7 is provided with an optical modulating signal generating circuit 42, and carries out the printing in four gradation levels including black and white in units of printing pixels. The optical section 2 and the image forming section 11 within a main body (not shown) of the laser beam printer has a construction similar to that shown in FIG. 1. An image memory 41 is made up of a 2-bit bit-map memory, and the image data are developed in advance in the image memory 41 by an image developing section (not shown) in a printable state. Since three gradation levels excluding white can be represented in units of printing pixels, three tone planes are prepared. For the sake of convenience, the tone of the printing pixel is represented by numerical values 0 to 3, with the larger numerical value representing a higher (darker) tone, the tone 0 representing white, and the tone 3 representing black.
The optical modulating signal generating circuit 42 is controlled to output the optical modulating signal in synchronism with the progress of the image write process in the main body, in response to a plurality of control clock signals which are generated by a control clock signal generating section 55 based on an optical scan timing signal which is output from the optical section 11.
The multi-value image data read from the image memory 41 by the image memory read section 43 is supplied to a tone decomposing section 45. The tone decomposing section 45 distributively outputs the binarized pixel data to a plane-1 correcting section 46-1 through a plane-3 correcting section 46-3 depending on a predetermined distribution rule, based on the tone of the pixel.
The plane-1 correcting section 46-1 through the plane-3 correcting section 46-3 carry out a smoothing process with respect to the input pixel data, and outputs a correction signal having a corresponding level. The binarized pixel data is input to each of the plane-1 correcting section 46-1 through the plane-3 correcting section 46-3. For this reason, the plane-1 correcting section 46-1 through the plane-3 correcting section 46-3 have the same construction as the corresponding section of the conventional monochromatic binary printing apparatus, and the same correction rule can be employed. In other words, it is possible to use the circuit shown in FIG. 3 as it is. The correction signals output from the plane-1 correcting section 46-1 through the plane-3 correcting section 46-3 are combined according to a predetermined combining rule in a correction signal combining section 52, and a combined output is supplied to a digital-to-analog (D/A) converter 53. An output of the D/A converter 53 is supplied to the laser diode of the optical section 2 as the optical modulating signal.
According to the predetermined distribution rule employed by the tone decomposing section 45, the pixel data is decomposed into tone planes for each of the pixel tones in the image memory 41 and distributed, for example. Hence, the pixel data having the tone 1 within the image memory 41 is distributed to the plane-1, the pixel data having the tone 2 is distributed to the plane-2, and the pixel data having the tone 3 is distributed to the plane-3. In addition, according to the predetermined combining rule employed by the correction signal combining section 52, if a plane in which the tone is other than zero exists with respect to the same printing pixel, for example, the correction signal of the plane with the highest tone is output with priority over others.
In the pattern shown in FIG. 8, the vertical lines have a jaggy portion at an intermediate portion. Vertical lines having low tones 1 and 2 in two stages are interposed between two vertical lines having a higher tone 3. In addition, a vertical line having a tone 2 is provided on the right side of the right vertical line having the tone 3. The data of this pattern is developed in the bit-map memory, that is, the image memory 41, as shown on the top left of FIG. 8. The pattern is distributed to each of the plane-1, plane-2 and plane-3 as shown in FIG. 8, and an independent smoothing process is carried out in each of the plane-1, plane-2 and plane-3. As a result, three corrected outputs are obtained as shown in FIG. 8. When the three corrected outputs are combined according to the predetermined combining rule described above, a printed result shown on the top right of FIG. 8 is obtained.
However, according to this multi-value smoothing method, if the number of gradation levels is increased so as to improve the half-tone representation and improve the printing quality, that is, if the number of output levels that can be output is increased, the number of tone planes to which the pixel data are to be decomposed increases. As a result, there was a problem in that the scale of the hardware of the smoothing circuit becomes large.